The present invention relates to improvements in hydrokinetic torque converters as well as to improvements in lockup clutches or bypass clutches which are utilized in torque converters. More particularly, the invention relates to improvements in torque converters of the character wherein a rotary housing defines a chamber for a pump, a turbine, a stator and a lockup or bypass clutch (hereinafter called lockup clutch), and wherein the clutch comprises an axially movable annular piston serving to divide the chamber into a first compartment and a second compartment. The chamber is filled with a fluid coolant (such as oil), and the piston is provided with or carries a first friction surface which can be moved into torque transmitting engagement with a second friction surface (which rotates with the housing) when the clutch is engaged. Still more particularly, the invention relates to improvements in hydrokinetic torque converters and lockup clutches wherein the first compartment is disposed between the piston and a component which carries the second friction surface and wherein the piston and/or the component is provided with one or more passages serving to establish one or more paths for the flow of fluid coolant from the second compartment (which accommodates the turbine, the pump and/or the stator of the torque converter) toward the first compartment.
U.S. Pat. No. 4,969,543 (granted Nov. 13, 1990 to Macdonald for "Slipping Bypass Clutch Construction For A Hydrokinetic Torque Converter") discloses a lockup clutch which comprises an annular piston provided with a first friction surface adapted to be moved against a second friction surface which is provided on a substantially radially extending wall of the housing of the torque converter. The piston or a friction lining on the wall of the housing is provided with several channels serving to permit a fluid to flow from the second compartment into the first compartment within the housing, even when the lockup clutch is engaged. The channels are provided at the same radial distance from the rotational axis of the housing as the friction surfaces, the first compartment of the chamber is disposed between the piston and the wall of the housing, and the second compartment serves to confine at least the turbine of the torque converter. The patentee attempts to prevent excessive thermal stressing of certain constituents of the torque converter; which thermal stressing could develop during continuous slipping of the friction surfaces of the lockup clutch when the torque converter is in use. More specifically, the patentee attempts to prevent excessive thermal stressing of the parts in the region of the friction surfaces of the lockup clutch.
Published Japanese patent application No. 58-30532 discloses a lockup clutch which is intended for use in torque converters and is also provided with channels in the region of the friction surfaces of the clutch.
The patent to Macdonald is but one of several publications which propose the utilization of a lockup clutch having friction surfaces which are permitted to slide relative to each other in the disengaged as well as in the engaged condition of the clutch. If a torque converter utilizing a lockup clutch proposed by Macdonald is installed in the power train of a motor vehicle, the slippage of the friction surfaces forming part of the lockup clutch can develop only during relatively short intervals of time (e.g., during shifting into a different gear) or it can be established and maintained at least substantially within the entire operating range of the torque converter. The extent and the duration of slip can be dependent upon the design of the prime mover which transmits torque to the housing of the torque converter and/or upon the selected gear ratio and/or upon one or more variable parameters of the prime mover. The lockup clutch dissipates energy in the form of heat during slippage of the friction surfaces with respect to one another, and the quantity of dissipated energy can be rather pronounced (e.g., in the range of several kilowatts) during certain stages of operation of the torque converter. Such circumstances can arise, for example, when a motor vehicle pulling a trailer is driven along a mountain road, and this can involve the dissipation of substantial amounts of energy for extended intervals of time. Furthermore, when the slip clutch is engaged, the amount of dissipated energy is likely to be greatly increased, at least for a relatively short interval of time, i.e., the lockup clutch and the torque converter are likely to be heated well above a permissible maximum temperature.
The purpose of the establishment of one or more paths for the flow of a fluid coolant (such as oil) is to prevent the aforediscussed drawbacks of presently known torque converters and their lockup clutches. A drawback of heretofore known undertakings to cool the torque converter in the region of its lockup clutch is that the rate of fluid flow along the friction surfaces of the lockup clutch is overly dependent upon the temperature and/or viscosity of the fluid and/or upon the differential between the fluid pressures at the opposite sides of the piston. In other words, if a torque converter and its lockup clutch are constructed and assembled in a manner as proposed, for example, in the aforediscussed patent to Macdonald, the resistance to the flow of the fluid in the channels between the two fluid-containing compartments must the selected in such a way that it is satisfactory under critical circumstances, namely the rate of flow of a fluid whose temperature has risen to a maximum possible or permissible value is less than the rate at which the system pressure in the torque converter would collapse or would drop to an unacceptably low value. In the torque converter which is proposed by Macdonald, the rate of fluid flow in the channels between the two compartments at the opposite sides of the piston of the lockup clutch is directly dependent upon the difference between the fluid pressures in the two compartments. Such pressure differential is that variable parameter which controls the transmission of torque by the lockup clutch and, therefore, it cannot be utilized or relied upon for the selection of the desired volumetric flow of the fluid. Therefore, and in order to maintain the losses in the torque converter above a minimum acceptable value, the rate of fluid flow must be low or relatively low even when the difference between the fluid pressures in the two compartments rises to a maximum value, namely when the torque converter is expected to transmit a maximal torque. This may ensure a satisfactory rate of flow of the fluid coolant when the converter is expected to transmit a maximal torque, but is unsatisfactory during transmission of lesser torque because the difference between the fluid pressures in the two compartments in the housing of the torque converter is too low.